1. Field of the Invention
The present invention relates to a color ink jet recording method and its apparatus for recording a clear and dense color image by discharging color inks from a recording head. In particular, the present invention concerns a color ink jet recording method and its apparatus using color inks of yellow (Y), magenta (M), cyan (C), green (G), red (R), blue (B) and the like and a black (Bk) ink.
2. Related Art
The ink jet recording method in which characters, figures and the like are recorded by discharging microdroplets of ink from discharge ports, has advantages in reducing the generation of noise, in lowering the running cost, in reducing the size of the apparatus, and in achieving the coloring. The method, therefore, has been extensively used for printers, copying machines, facsimile and the like.
In a prior art ink jet recording method, a sheet of special paper (coated paper) having an ink absorbing layer has been necessary to obtain a color image being high in color development with little bleed. Recently, a method has been practically used having a suitability for printing on sheets of commercially available "plain paper" such as high-grade/medium-grade paper and common PPC paper used for printers and copying machines in large amounts. However, in the case of color recording on a sheet of plain paper, sufficient printing quality cannot be obtained yet. In this case, the largest problem lies in that the prevention of bleed of ink between color inks used is incompatible with the insurance of the quality of black recording, particularly, the quality of black character recording.
In recent years, to form a color image with little bleed on plain paper by an ink jet recording method, a quick drying ink having a high permeating rate against plain paper has been developed. In the color ink jet recording method, color recording is performed using inks of three colors, cyan, magenta and yellow, and further, using inks of three colors, cyan, magenta, yellow, and a black ink. In the case of using the above-described inks excellent in permeability for color ink jet recording, the recorded image has high quality with little bleed between colors but is low in density as a whole, and further in the surroundings of the recording image area of each color, the ink is slightly bled along fibers of the paper (feathering), resulting in poor sharpness.
The feathering is relatively inconspicuous in a color image area; however, it is conspicuous in a black image area, thus deteriorating the recording quality. In particular, for a black character, it is poor in sharpness and unclear, resulting in the poor recording quality.
Accordingly, to obtain a high quality black color recording with reduced feathering and higher density, a black ink used for forming a black image must be of a type having a relatively low permeating rate against plain paper.
For this reason, there has been proposed an ink jet recording apparatus and its method using color inks each having a high permeating rate against paper and a black ink having a low permeating rate against paper. However, in the case of using the inks each having a high permeating rate and the ink having a low permeating rate, at the adjacent boundary portion between a black recording image area and each color recording image area, irregular blur is generated, thus significantly reducing the image quality.
The prevention of irregular blur between black and each color is incompatible with the reduction in feathering of black, and therefore, as the subject of the color recording, it has been required to simultaneously achieve the prevention of bleed and the reduction in feathering of black.
Unexamined Japanese Patent HEI 3-146355 discloses a method, wherein a recording area along the boundary area between black and each color is not recorded; however, this method is disadvantageous in that the recorded data is changed.
Another method of preventing bleed of ink in a boundary area between black and each color has been known, wherein a black area along the boundary area between black and each color is formed by overlapping of color inks. In this specification, the black color thus formed is called Process Color Bk (hereinafter, abbreviated as "PCBk").
In the method of forming the black area of PCBk along the boundary area between black and each chromatic color, it is easiest to replace all black data with PCBk data of Y, M and C. Since the PCBk is formed of only inks of Y, M and C, each of which is high in permeability, it is not bled at the boundary portion between black and each color. However, the PCBk is very different from the black color formed of the Bk ink in terms of hue, and thereby it is fringed by the boundary portion. For this reason, to adjust the hue of the PCBk, an attempt has been made to add the Bk ink in the black area formed of PCBk to some extent.
However, when an ink having a low permeability is adjacent to an ink having a high permeability, a blurry white image (in this specification, referred to as "white haze") is generated at the adjacent boundary portion, as shown in FIG. 7, which greatly exerts adverse effect on the image quality.
The "white haze" is considered to be generated by the following mechanism. The mechanism will be described with reference to FIGS. 8(a) to 8(c).
An ink having a low permeability is low in wettability against paper, and is generally high in surface tension. On the contrary, an ink having a high permeability is high in wettability against paper and is low in surface tension, and in general, it contains as an additive a surface-active agent for improving the wettability against paper. FIG. 8 shows an example wherein black ink has a low permeability and color ink has a high permeability. When an area (color ink area) recorded by the ink containing the surface-active agent for improving wettability is adjacent to an area (black ink area) recorded by the ink with no surface-active agent, as shown in FIG. 8(a), the surface tension of ink with no surface active agent at the adjacent boundary is lowered by the action of the surface-active agent, and further the ink with no surface-active agent is applied with internal cohesive force because of its high surface tension and tends to be moved in such a manner as to be formed in a spherical shape. Consequently, as shown in FIG. 8(b), the surface-active agent is spread even along the area recorded by the ink with no surface-active agent, and accordingly, when being fixed in such a state, an area with less dye is generated as shown in FIG. 8(c) and is recognized as the "white haze".
As a result, although the bleed of ink at the boundary area between different colors can be prevented, the image is observed as being poor by the generation of the above-described "white haze".
One example of a combination of ink compositions used in the case where the "white haze" is generated, is shown as follows:
______________________________________ 1. Y (yellow) C. I. Direct Yellow 86 3% diethylene glycol 10% isopropyl alcohol 2% urea 5% "Acetylenol EH" 1% (sold by Kawaken Fine Chemical) water 79% 2. M (magenta) C. I. Acid Red 289 3% diethylene glycol 10% isopropyl alcohol 2% urea 5% "Acetylenol EH" 1% (sold by Kawaken Fine Chemical) water 79% 3. C (cyan) C. I. Direct Blue 199 3% diethylene glycol 10% isopropyl alcohol 2% urea 5% "Acetylenol EH" 1% (sold by Kawaken Fine Chemical) water 79% 4. Bk (black) C. I. Direct Black 154 3% diethylene glycol 10% isopropyl alcohol 2% urea 5% water 80% ______________________________________
Each color ink (C, M, Y) has a low surface tension of about 30 dyn/cm, and a high blotting factor against plain paper (diameter of a dot formed on a recording sheet/diameter of a discharged ink droplet) of nearly from 2.5 to 3.5, and accordingly, it is easier to be spread and slightly lowered in sharpness and to be permeated in paper. On the other hand, the Bk ink has a high surface tension of about 50 dyn/cm, and a low blotting factor against plain paper of nearly from 1.7 to 2.0, and accordingly, it is difficult to be spread and to be permeated in paper.